Iodine-123 and Iodine-131 radioisotopes are presently used in medical diagnosis and radiation therapy. Meta-iodobenzylguanidine sulfate labelled with Iodine-123 and Iodine-131 has been used clinically in the diagnosis and treatment of pheochromocytomas, neuroblastomas and other paragangliomas.
For many years, Iodine-124 was considered to be a troublesome radiocontaminant which increased the absorbed radiation dose of the patient and detracted from the otherwise high quality scintigraphs that could be obtained with high purity Iodine-123 However, Iodine-124 decays by positron emission and can therefore be used in positron emission tomography ("PET"), a recently developed state-of-the-art technology, and Iodine-124 can thereby be used for non-invasive quantitative physiological studies. For example, when meta-iodobenzylguanidine (m-IBG) is labelled with Iodine-124 (Iodine-124-m-IBG), it is useful in obtaining quantitative images of the brain, adrenal, and myocardium when used in conjunction with PET technology.
Iodine-124 has a physical halflife of about 4.16 days. This isotope provides medically useful positron nuclear emissions of about 25 per 100 nuclear decay events of Iodine-124. The positrons, which have a maximum end point energy of 2.1 MeV, interact with matter and annihilate into two photons of about 511 keV energy at about 180 degree referenced to the point of annihilation. The annihilation quanta are readily detected by PET instruments.
Mathematical methods can be used to reconstruct the volume element in which the radioactive decay process occurred Since the volume element in which the Iodine-124 decayed can be defined with mathematical models, it is possible to quantitatively measure regional physiological parameters, such as blood flow, metabolism, tissue pH, and receptor specific interactions.
Since radioactivity can be quantitated within a given number of pixels (volume elements), it is possible to define the size and shape of the profile of distribution of Iodine-124. This enables a more accurate staging of the appropriate therapeutic dose of internal delivered radioactivity applied for patient treatment.
A radioactive iodine (radioiodide) isotope in conventional use is Iodine-131 with a halflife of about 8.1 days and which decays be emission of beta particles and various gamma emissions. The beta radiation is utilized in therapy, such as when Iodine-131 iodide is used for the treatment of thyroid carcinoma. The radiation dose delivered from Iodine-124 is approximately 69% of that delivered by the Iodine-131 radionuclide generally used in internal radiotherapeutic applications.
For conventional diagnostic tests, an ideal radioiodide isotope is Iodine-123, which has a physical halflife of 13.1 hours and decays by a high abundance of 159 keV gamma rays. The absorbed radiation dose per unit of the injected dose of "pure" Iodine-123 is 1/100th of the radiation dose associated with Iodine-131.
Iodine-121 and Iodine-122 have been suggested as appropriate medical radiohalogens. However, both are limited by the physical halflife of 2.1 hour and 3.5 minutes, respectively, compared to 4.12 day halflife of Iodine-124
Certain iodinated radiopharmaceuticals require a radio-isotopic label with minimum halflife of 0.5 to 1.0 days. Iodine-124 is therefore an important radiohalogen. The major problem encountered in the application of Iodine-124 to PET has been obtaining the Iodine-124 in sufficient production yields an radionuclidic purities.
A known method of producing Iodine-124 is by Tellurium-124(p,n)Iodine-124 reaction, disclosed in Kondo et al., 28 Int. J. App. Rad. and Isotopes 765 (1977). However, this reaction is not efficient and typically results in low yields.
Consequently, an object of this invention is to provide a method of obtaining Iodine-124 having sufficient production yield and radionuclidic purity for use with PET instrumentation.
A further object of this invention is to provide a method for producing Iodine-124 at levels appropriate for commercial sales, either as a precursor or as a labelled pharmaceutical.
A further object of this invention is to provide a method of producing Iodine-124 with consistent purity so that it can be used as a radioactive standard for calibration of radiologic equipment.
A further object of this invention is to provide a method of producing Iodine-124 which is safe and reliable.
Other objects and features of this invention will become apparent to those skilled in the art after reviewing the following specification.